In general computed tomography apparatus (CT) for medical purposes shown in FIG. 1, a CT beam, which is transmitted by CT beam transmittance part 10, passes through an object 30 and signals passing through the object are detected by a detector, so that the object is restored as three-dimensional images by using the signals to output them to an user.
In case of the above manner, since the three-dimensionally restored object is randomly cut and only the cut planar images are checked, it cannot effectively calculate of the volume of a specific part existed in the inside of the photographed object.
The CT is widely used in medical. Also, it is widely used to diagnose the internal structure of the human body. Moreover, the utilization thereof for observing the defective parts such as the internal structure of the product, the internal inclusions or internal cracks in the field of industrial has been increased.
In particular, recently, it has begun to introduce the CT in the field of the geological resource. One of its main purposes is to observe the internal characteristics of the samples obtained from the strata.
Any gap generated from the crack of the strata or the gap between the particles of constituting the strata which is called as a pore.
Since the useful substances such as oil, gas, and groundwater can be flowed smoothly through these pores in the stratum, it is important to identify quantitatively the amount of the pores in the strata through the analysis of samples obtained from the strata.
The amount of the pores in the strata is mainly expressed as a population parameter called as a porosity, which is represented by the following formula.Porosity(%)=the amount of the pores of sample/total volume of the sample*100  (formula)
There are an immersion process, a gas process, and a mercury process etc. in a method for measuring the pores from geological samples. In the above methods, a water, a gas, and a mercury are filled into or discharged out the sample or discharged to measure the quantity of demand.
In these methods, accessory materials such as water, helium gas, mercury etc. are required and appropriate instrument should be utilized. Also, it takes a considerable amount of time to measure the sampling unit. Accordingly, it is necessary to save the time and expense. Also, the necessity of developing accurate measuring method of the porosity has been brought up.
There are continuous attempts for observing the internal structure of any particular substance using the CT. Recently, there is an attempt for identifying and quantifying the volume of specific substances in the strata sample.
That is, there is an attempt for easily measuring the porosity thereof by analyzing the strata sample through the CT. However, a highly reliable method for measuring the porosity has not been presented yet.
Where the porosity is measured through the CT analysis, the numerical values read in sectional image of the CT image are variable.
If the sample of the geological resource having the same property is collected in the same strata, as though the CT scan on it is progressed in the transceiving conditions of the CT beam, in case of different sizes thereof, the gray level values of representing the pores of the sample can be only different from each other.
Also, in case of representative samples of the geological resource, the groundwater, the oil, or the gas is filled in the pores. However, the gray level values of representing the pores of the sample in the CT sectional image can be only different according to the kind of the filling material of the pore.
By scanning the standard sample and the sample to be measured (hereinafter, referred to as the measurement sample is defined) together by means of the CT, a system for accurately measuring the porosity of the measurement sample using the numerical values read from the sectional images of the standard sample has been demanded.